Fabrication of dark heaters

ABSTRACT

In the dip coating of aluminum oxide coated heaters to provide an outer dark coating consisting essentially of tungsten, a coating bath is used which is substantially rheologically stable, i.e., one in which the rate of settling of the tungsten particles is extremely slow.

ijite I States atent 1 1 Hale 1 Dec. 3 1974 [54] FABRICATION 0F DARK HEATERS 3,328,201 6/1967 Scheible 117 217 340127 1968 F'l'b ..ll7 1 [75] Inventor: John Richard Hale, Lancaster, Pa. 9 em e1 ,2 7 73 Assigne'e: RCA Corporation, New York, N.Y. FOREIGN PATENTS OR APPLICATlONS [22] in d A 7 1972 1,060,942 3/1967 Great Britain 1e pr.-

[21 Appl. No.: 242,240 Primary Exa'miner Cameron K. Weiffenbach Attorney, Agent, or FirmG. H. Bruestle; L. 52 us. c1 117/217, 117/227, 117/231, GreenspaniM' 313/345 [51] Int. Cl -B44d 1/18 57 ABSTRACT [58] Field of Search 117/227, 217, 31, 29, 231;

313/345 In the dip coating of alumlnum oxide coated heaters to provide an outer darkcoating consisting essentially [56] References Cited of tungsten, a coating bath is used which is substanv tially rheologically stable, i.e., one in which the rate of UNITED STATES P T settling of the tungsten particles is extremely slow, 2,764,511 9/1956 lverscn 313/345 3,246,197 4/1966 Watkins 313/345 4 Claims, N0 Drawings FABRICATION OF DARK HEATERS BACKGROUND OF THE INVENTION This invention relates to electron discharge tubes, and particularly to a method of fabricating dark insulated heaters for such tubes.

In various types of electron tubes having indirectly heated cathodes, it is the practice to use dark heaters to heat the cathode to electron emitting temperatures. Such heaters comprise a core wire of a refractory metal, such as tungsten, a first wire covering layer of an insulating material, such as aluminum oxide, and an outer dark coating such as a particulate mixture of tungsten and aluminum oxide. A purpose of the first coating is to provide insulation between the heater wire and the cathode, and a purpose of the outer coating is to increase'the thermal emissivity of theheater, thereby lowering the temperature at which the heater need operate to heat the cathode to its operating temperature.

One problem with-the use of insulated heaters, in general, is that in some electron tubes, such as color picture tubes, a too frequent cause of tube failure is electrical arcing between the heater and cathode of the tube. The cause of such arcing, it is believed, is that where the particulate and relatively rough insulating coating of the heater makes comparatively small area or point" contact with the cathode. Such high potential gradients give rise to breakdown of the heater insulation and arcing between the heater and cathode. Since the outer coating of a dark heater is essentially an insulator, owing to the factthat the tungsten particles are dispersed among aluminum oxide particles, this problem of arcing exists in tubes using dark heaters of the type described.

It has been demonstrated that the use of a heater outer coating consisting entirely of a conductive material, e.g., tungsten, significantly reduces the arcing problem regardless-of the type of contact between the heater outer coating and the cathode. This follows because the conductive coating and the cathode are at the same potential, whereby no potential gradient exists therebetween. While-a potential difference exists between the heater outer coating and the heater core wire, the first layer of insulating material provides a generally uniform spacing between the two, thereby avoiding the creation of non-uniform and excessively high potential gradients.

While the use of insulated heaters having an outer coating consisting entirely of tungsten has been suggested in the past (see, for example, US. Pat. No. 3,195,004 issued to Hassett on July 13, 1965), the use of such heaters has generally not been; practical. One reason for this is that, in the past, as the percentage of tungsten in the outer coating was increased, the

amount of current leakage between. the heater and the cathode increased. This is generally undesirable, and the practice inthepast has been to' limit the ratio of tungsten to aluminum oxide in the outer coating'to some upper limit, e.g.,'in the order of 40 percent.

DETAILED DESCRIPTION OF THE INVENTION One method generally known for applying various coatings to heater structures is known as dip coating. This method involves dipping the heater to be coated into and out of a bath containing a suspension of the particularly high potential gradients frequently exist particles to be coated on the heater, the particles adhering to the heater as the emerging heater breaks through the surface of the bath. If the heater structure is in the form of a continuous wire or the-like which is drawn through the bath, the process is known as a drag coating process. The thickness of the coating is a function of the specific gravity and viscosity of the bath.

As previously noted, a problem in the past 'using dark heaters havinglarge percentages of tungsten in the outer coating is that the tubes using the heaters generally have unacceptibly high levels of heater to cathode leakage. The cause of such leakage is that particles of tungsten from the outer coating somehow penetrate into the aluminum oxide undercoating and provide leakage paths for current through the undercoating.

The higher'the percentage of tungsten in the outer coating, the greater the amount of tungsten that penetrates into the undercoating.

After investigation by myself, I discovered a principal cause of the tungsten penetration. During the clipping operation, according to prior art processes, .relatively rapid settling of the high density tungsten particles occurs within the bath, with the result that the ratio of tungsten to liquid vehicle near the surface of the bath rapidly decreases. As this ratio decreasesmore and more, the fluidity of the bath near the bathsurface increases to a point where the viscosity of the bath is so low that it' readily penetratesthe layer of aluminum oxide. Tungsten particles are conveyedwith the bath into thealuminum oxide undercoating, thereby providing conditions for leakage in tubes using the heaters. Heretofore, by significantly limiting the amount of tungsten in the bath, the adverse effects of penetration of the bath into the underlying layer of aluminum oxide was minimized. However,.even with small amounts of tungsten in the bath, the useof'dark heaters in certain tube.

types in which a high voltage is applied between the heater and cathode, such as picture tubes used in television receivers, has not been as extensive as might otherwise be desired.

A solution to this problem of penetration, I discovered, is the use of a bath in which the viscosity at the surface of the bathis and'remains' high during a reasonably long time adequate to perform the dipping operation. Stated differently, what is required is a bath which is substantially rheologically stable, i.e., one in which the rate of settling of the particles of the bath is so slow as to retain a high viscosity at the surface thereof. While, theoretically, all that is required is a bath which is stable only during thetirrie it takes to dip a heater into and out of it, as a practical matter, it is preferred that the bath be so stable as to allow repeated use thereof over a period of hours without constant restirring and reconstitution.

To obtain such a practically useful, rheologically stable bath, I'discovered, requires the use of a bath in which there is a high degree of dispersion of the particles within the bath, i.e., a full separation of the particles from one another, and 'a complete wetting of each particle by theliquid phase.

Various factorsof importance with respect to obtaining full particle dispersion in a coating bath containing tungsten particles only, i.e., with no aluminum oxide, are now discussed.

While it is possible to purchase on the market various grades of tungsten powder, the cost of the tungsten matcrial generally rapidly increases with decreasing particle size and particle size range. Thus, for reasons 6f cost, it is desirable to use tungsten material of large particle size for the coating bath. Conversely, for obtaining good control over the thickness of the coating applied to the heater, and providing good continuity of the applied coating for thin coats of tungsten, small particle size is desired. A satisfactory compromise is to use commercially-available tungsten predominantly in the particle size range of 0.5 to 10 microns, with the average particle size being about 2 microns.

In connection with this discussion concerning particle size, it is noted that with this selected range of particle size, it is not to be expected that stable, sol-like suspensions can be obtained. That is, true sols, or colloidal suspensions, contain particles of a size generally no greater than 0.2 micron. Moreover, aside from the comparatively great expense associated with the use of tungsten particles of such small size to prepare such a colloidal bath, the coating characteristics of such a bath would be quite impractical, i.e., dipping in the bath would result in minimal coating of the tungsten onto the heater. In spite of the relatively large tungsten particles used, however, I have found that by providing full dispersion of the tungsten particles, substantially rheologically stable baths having good coating characteristics can be provided.

With respect to obtaining such full dispersion, it is noted that commercially available tungsten powder generally comprises relatively large agglomerates or tightly packed clusters of individual particles of tungsten. One means used in the past for deagglomerating the tungsten powder, in the fabrication, for example, of dark heaters comprising outer coatings of aluminum oxide mixed with tungsten particles, comprises dry or wet rolling the tungsten powder in a mill or the like with no milling media other than the tungsten itself. I discovered that in order to obtain an adequately stable bath, substantially complete deagglomeration of the tungsten powder is necessary, and that the prior art technique of rolling is totally inadequate.

To adequately deagglomerate the tungsten powder, a relatively high shearing force must be applied to the individual tungsten particles. For example, external agitation of adequate force can be applied by some suitable mechanical means, e.g., the commercially available Kady Mill sold by Kinetic Dispersion Corp., Buffalo, N.Y. However, material and solvent loss occurs during dispersion. Alternatively, ultrasonic vibration of the bath can be used. This, however, requires a rather large and expensive system, and vehicle breakdown occurs.

Probably the simplest and most economical means for achieving adequate deagglomeration, l have found, is the use of a milling process employing, for example, a high density alumina milling jar with tungsten carbide milling media. Although not critical, I find that preferred results are achieved using relatively large media, e.g., 3/16 inch long, round ended cylinders. The use of such a milling operation is especially desirable since it causes no breakdown of the liquid vehicle and no adverse contamination of the bath, the materials of both the mill and the media being quite compatible with the heater materials.

The proper duration of the milling operation for a given bath to provide complete dispersion of the tungsten particles can be determined by several means.

For example, until complete dispersion of the tungsten particles occurs, the apparent specific gravity of the bath, as measured with a hydrometer, is less than the true specific gravity of the entire bath, as determined by dividing the weight of the entire bath by the volume of the bath. The reason for this is that the incompletely deagglomerated clusters very rapidly settle to the bottom of the bath and do not provide buoyancy for the hydrometer. As full dispersion conditions are reached, however, the apparent specific gravity very nearly approaches, and often just about equals, the true specific gravity.

The stability of the resulting suspension can be conveniently defined in terms of the change in the apparent specific gravity with time. For example, using a bath comprising (by weight) 40 percent tungsten and percent liquid vehicle, the vehicle comprising 1.68 percent 1,000. second nitrocellulose wet 30 percent by weight by alcohol and 98.5 percent butyl acetate, after ball milling for about 6 hours, the change in apparent specific gravity of the suspension, owing to settling of the tungsten, is only in the order of 2 percent over a period of 8 hours. As a practical matter, for practical utility in a normal mass production continuous heater coating process, a rheologically stable bath is defined as one in which the change in apparent specific gravity of the bath is less that5 percent in 8 hours.

The use of such a bath is especially desirable in automated dipping operations because the bath is so stable as to require little attention, i.e., stirring, during its useful life. The life of the bath is generally determined by the rate of evaporation of the liquid phase, such evaporation changing the coating characteristics of the bath. With the'above-described bath, the life is about 8 hours. Penetration of tungsten into the underlying layer of aluminum oxide is substantially avoided as long as the dipping operation is performed while the apparent specific gravity of the bath is still high, e.g., within 5 percent of the true specific gravity.

Another method of determining when complete dispersion is reached is by monitoring the viscosity of the bath during the milling operation. Maximum dispersion of the particles corresponds to maximum viscosity of the bath. This follows because maximum dispersion of the particles within the liquid phase gives rise to maximum particle-to-particle interaction, i.e., maximum internal friction. Again, penetration of the tungsten into the underlying layer of aluminum oxide is substantially avoided as long as the dipping operation is performed while the viscosity of the bath near the surface thereof is high, e.g., within 5 percent of the maximum bath viscosity. By near the surface is meant within a few mils of the surface.

The degree of dispersion can also be measured using a commercially available fineness of grind meter," e.g., the Nipri Type G-l Meter, sold by Precision Gauge and Tool Company, Dayton, Ohio. Maximum dispersion of the tungsten particles is reached when further milling does not result in a finer aggregate particle size.

As previously noted, the thickness of the tungsten coating applied to the heater in the dipping operation is a function of, among other things, the viscosity and specific gravity of the bath. Depending on the particular heater being made, different coating thicknesses are desired, hence different baths having'different viscosities and specific gravities are used.

An equation governing the viscosity of a suspension of tungsten particles whose particle size range has an upper limit of IO microns, with an average size of 2.0 microns. in a liquid vehicle is as follows:

1y=no(l +182 (1)) where:

1; is the viscosity of the resulting system,

no is the viscosity of the liquid phase, and

(b is the ratio of the volume of the tungsten material in the dry powder state to the sum of the volume of the dry tungsten powder plus the volume of the liquid phase.

The above equation is valid for tungsten particles of the specified size range and after full dispersion conditions are achieved, as by adequate ball milling of the suspension. Variations in the viscosity of the liquid phase (170) and the particle size range affect the value of the constant associated with the qb factor. This can be developed experimentally.

The following Table provides data concerning different baths for use in providing different thickness coatings of tungsten ina dipping operation.

While the invention has been described in connection with outer coatings consisting entirely of tungsten, the invention also has utility in providing heaters having outer coatings comprising tungsten and some other insulating material, generally aluminum oxide. By providing a rheologically stable bath, as by ball milling both the tungsten particles and the other insulating material in a suitable vehicle, penetration of the outer coating particles into the underlying layer of oxide is avoided.

By way of example, in any of the baths listed in the aluminum table, a portion of the tungsten can be replaced with aluminum oxide, the same liquid vehicle being used. The bath is ball milled. using a tungsten carbide milling media, until maxium conditions of apparent specific gravity and viscosity are achieved.

I claim:

1. A method of providing a coating on a heater structure comprising:

preparing a substantially rheologically stable bath by dispersing tungsten particles in a liquid vehicle until the apparent specific gravity of said bath, as measured by a Hydrometer, is substantially equal to the true specific gravity of the bath taken as a whole, and then passing said heater into and out of said bath within a period of time in which the difference between the apparent and true specific gravities of said bath is not greater than 5 percent. 2. The method of claim 1 wherein said bath preparing step comprises ball milling said bath using a tungsten carbide milling media.

3. The method of claim 2 wherein said bath consists essentially of, by weight, tungsten, in the range of 30 to 60 percent, said tungsten being provided as a powder having a particle size in the approximate range of 0.5 to 10 microns.

4. A method of coating an aluminum oxide coated heater with an outer layer consisting essentially of tungsten comprising:

preparing a substantially rheologically stable bath consisting essentially of, by weight, tungsten, in the range of 30 to 60 percent, and a liquid vehicle, in the range of 40 to. percent, said tungsten being provided as a powder having a particle size in the.

- approximate range of 0.5 to 10 microns,

ball milling said bath in an alumina mill using tungsten carbide milling media until the apparent specific gravity ofsaid bath as measured by a hydrom eter is substantially equal to the true specificgravspecific gravities is not greater than 5 percent. 

1. A METHOD OF PROVIDING A COATING ON A HEATER STRUCTURE COMPRISING: PREPARING A SUBSTANTIALLY RHEOLOGICALLY STABLE BATH BY DISPERSING TUNGSTEN PARTICLES IN A LIQUID VEHICLE UNTIL THE APPARENT SPECIFIC GRAVITY OF SAID BATH, AS MEASURED BY A HYDROMETER, IS SUBSTANTIALLY EQUAL TO THE TRUE SPECIFIC GRAVITY OF THE BATH TAKEN AS A WHOLE, AND THEN PASSING SAID HEATER INTO AND OUT OF SAID BATH WITHIN A PERIOD OF TIME IN WHICH THE DIFFFERENCE BETWEEN THE APPARENT AND TURE SPECIFIC GRAVITIES OF SAID BATH IS NOT GREATER THAN 5 PERCENT.
 2. The method of claim 1 wherein said bath preparing step comprises ball milling said bath using a tungsten carbide milling media.
 3. The method of claim 2 wherein said bath consists essentially of, by weight, tungsten, in the range of 30 to 60 percent, said tungsten being provided as a powder having a particle size in the approximate range of 0.5 to 10 microns.
 4. A method of coating an aluminum oxide coated heater with an outer layer consisting essentially of tungsten comprising: preparing a substantially rheologically stable bath consisting essentially of, by weight, tungsten, in the range of 30 to 60 percent, and a liquid vehicle, in the range of 40 to 70 percent, said tungsten being provided as a powder having a particle size in the approximate range of 0.5 to 10 microns, ball milling said bath in an alumina mill using tungsten carbide milling media until the apparent specific gravity of said bath as measured by a hydrometer is substantially equal to the true specific gravity of the entire bath as determined by dividing the weight of the entire bath by the volume of the bath, and passing said heater into and out of said bath of coating tungsten onto the outer surface thereof, said coating being done near the surface of said bath while the difference between the apparent and true specific gravities is not greater than 5 percent. 